offline updates and playhead fix

lib/playback/playback_engine.dart

@@ -263,7 +263,17 @@ class PlaybackController extends StateNotifier<PlaybackState>
       (event) {
         _posAnchor = event.updatePosition;
         _posAnchorAt = event.updateTime;
-        state = state.copyWith(position: event.updatePosition);
+        // `updatePosition` is sampled at `updateTime`, i.e. slightly in the
+        // past. While playing, the ticker has already advanced the displayed
+        // position to ~now; writing the raw sample here would snap it backward
+        // every time an event fires (they fire periodically), then the ticker
+        // re-advances it — a visible flicker. So reflect the *interpolated*
+        // value (continuous with the ticker) while playing, and the raw value
+        // only when paused/buffering, where it's exact and nothing is ticking.
+        final pos = (state.playing && !state.buffering)
+            ? _interpolatedPosition()
+            : event.updatePosition;
+        state = state.copyWith(position: pos);
       },
       onError: (Object e, StackTrace st) => _onPlayerError(e),
     );
@@ -281,12 +291,20 @@ class PlaybackController extends StateNotifier<PlaybackState>
   /// the Subsonic metadata length) so it can't run past the end.
   void _tickPosition() {
     if (!state.playing || state.buffering) return;
+    state = state.copyWith(position: _interpolatedPosition());
+  }
+
+  /// The current position interpolated off [_posAnchor] against the wall clock,
+  /// clamped to [PlaybackState.effectiveDuration] (falls back to the Subsonic
+  /// metadata length) so it can't run past the end. Shared by the ticker and
+  /// the event listener so both agree — a mismatch between them is what causes
+  /// the playhead to visibly jump.
+  Duration _interpolatedPosition() {
     final elapsed = DateTime.now().difference(_posAnchorAt);
-    if (elapsed.isNegative) return;
-    var pos = _posAnchor + elapsed;
+    var pos = elapsed.isNegative ? _posAnchor : _posAnchor + elapsed;
     final total = state.effectiveDuration;
     if (total > Duration.zero && pos > total) pos = total;
-    state = state.copyWith(position: pos);
+    return pos;
   }
 
   /// Id of the song we last ran play side effects for. Queue edits shift